The semiconductor industry has a continuing interest in measuring thin films formed on wafers. A number of metrology devices have been developed for making these measurements. Many of these devices rely on probing the sample with a beam of radiation having a wavelength in the visible and/or UV regions. These devices work quite well on many different types of films which are at least partially transparent at these wavelengths. Unfortunately, these devices are not effective for investigating opaque and metal films, since opaque and metal films (such as copper) do not transmit either UV or visible radiation.
There have recently been developed some techniques using wavelengths in the X-ray regime. These X-ray reflectometry techniques (XRR) have several advantages over techniques using visible light. One such advantage is that XRR makes it possible to measure the thickness of ultra-thin films whose thicknesses are on the order of 30 angstroms or less. Visible light is not suitable for the study of such ultra-thin films using interference patterns because of its wavelength. However, an XRR system may preferably use radiation at wavelengths of about 1.5 angstroms, which radiation creates suitable interference patterns even when probing such ultra-thin films. In addition, XRR may suitably be used where the film is composed of a material that is opaque to light, such as a metal or metal compound. Another possible application for XRR methods might be as an in-situ monitor where only a grazing angle beam of radiation can be used to monitor a sample in a process chamber. Finally, XRR may suitably be used to measure the density and thickness of films composed of materials that have a low dielectric constant and a correspondingly low index of refraction, such as certain polymers, carbon fluoride compounds, and aerogels.
A preferred XRR technique is described in U.S. Pat. No. 5,619,548, issued Apr. 8, 1997, which is hereby incorporated by reference in its entirety. FIG. 1 illustrates this preferred technique. (See also Japanese Patent No. 2,720,131, issued Nov. 21, 1997.)
Referring to FIG. 1, the preferred X-ray scattering system includes an X-ray source 31 producing an X-ray bundle 33 that comprises a plurality of X-rays shown as 35a, 35b, and 35c. An X-ray reflector 37 is placed in the path of the X-ray bundle 33. The reflector 37 directs the X-ray bundle 33 onto a test sample 39 held in a fixed position by a stage 45, and typically including a thin film layer 41 disposed on a substrate 43. Accordingly, a plurality of reflected X-rays, 57a, 57b, and 57c (forming bundle 55) concurrently illuminate the thin film layer 41 of the test sample 39 at different angles of incidence. The X-ray reflector 37 is preferably a monochromator. The diffraction of the incident bundle of X-rays 33 within the single-crystal monochromator allows only a narrow band of the incident wavelength spectrum to reach the sample 39, such that the Bragg condition is satisfied for this narrow band. As a result, the plurality of X-rays 57a, 57b, and 57c, which are directed onto the test sample 39, are also monochromatic. A detector 47 is positioned to sense X-rays reflected from the test sample 39 and to produce signals corresponding to the intensities and angles of incidence of the sensed X-rays. A processor 60 is connected to the detector to receive signals produced by the detector in order to determine various properties of the structure of the thin film layer, including thickness, density and surface roughness.
In a basic system, a probe beam of X-ray radiation is directed to strike the sample at an angle selected so that it is at least partially reflected. A sample may typically consist of a substrate covered by one or more thin metal layers. At very shallow angles, below a critical angle (xcexa8c) (as measured between the surface of the sample and the incoming ray), all the X-ray radiation will be reflected. As the angle of incidence of the incoming beam increases with respect to the sample surface, an increasing amount of radiation will be transmitted through the top metal layer and the amount of reflected light will be reduced. Some of the radiation transmitted through the metal layer(s) will reach the interface between the metal film and the substrate and be reflected off the substrate.
The radiation reflected at the interfaces among the metal film layer(s) and the substrate will interfere, giving rise to a reflectivity curve showing interference effects. FIG. 2 shows two such angular reflectivity spectrums. The top spectrum curve S2 is for a tantalum layer on a substrate and the bottom spectrum curve S1 is for copper on a substrate. This variation gives rise to the appearance of interference fringes 18 in the measured signals S1 and S2. Since the reflection coefficient is so small (typically much less than one), multiple reflections have a relatively undetectable effect on the X-ray reflection signal. In both curves, the reflectivity decreases rapidly with increasing xcexa8.
In practice, the angle of incidence of the X-rays can be varied by moving the X-ray source, or by tilting the sample. In the prior art cited above, multiple angle of incidence X-rays can be created by focusing the source radiation, which functions to bend the rays within the beam to strike the sample at different angles of incidence.
The concept of the subject invention is most preferably employed with a simultaneously multiple angle of incidence embodiment, the Rapid X-ray Reflectometry or xe2x80x9cRXRRxe2x80x9d approach, although it can be used in the more convention al X-ray reflectometry or XRR approach, which requires actively varying the angle between the source and the sample.
The subject invention can also be applied to energy dispersive techniques in which a broad spectrum of X-ray energies are applied at a fixed angle. Such broad spectrum X-ray radiation may suitably be generated by the Bremsstrahlung radiation of a rotating anode. The X-ray reflectivity is then measured at each energy. Such an energy dispersive X-ray technique is described in Chason et al., Phys. Rev. Lett. 72, 3040 (1994) and Chason et al., Appl. Phys. Lett. 60, 2353 (1992), each of which is hereby incorporated by reference in its entirety.
Measurement of metal layers is very difficult on semiconductor patterned wafers. A typical measurement spot size for XRR or RXRR is one millimeter or larger. Since the feature sizes on a patterned wafer are on the order of one micron, and since even test sites on a patterned wafer have dimensions typically smaller than 100 microns, the accurate determination of single or two-layer metal thicknesses on a patterned wafer was believed to be very difficult.
The approach described herein provides the capability for measuring the thickness of one, two or even more layers (metal, opaque or dielectric) on patterned wafers while still using a one millimeter spot size which is larger than the feature size on the patterned wafer.
The teachings of the subject invention lead to a new application of the XRR and RXRR systems. In particular, it has been recognized for the first time that such systems can be used to measure thickness of a variety of thin films (both dielectric, opaque and metal films) on patterned wafers where the feature size is smaller than the measurement spot. Broadly speaking, one aspect of the invention is the recognition that XRR and RXRR systems can be used not only on test wafers but on patterned wafers as well. The specific teachings herein are intended to help simplify and enhance this basic concept. However, it should be understood that prior to this disclosure, we are not aware that anyone has attempted to use such systems on patterned wafers with small feature sizes.
FIG. 5 illustrates a patterned wafer 20 consisting of a silicon substrate 22, dielectric layer 24 and metal layer 26. As can be seen, the thickness of the metal layer 26 is constant, but the oxide layer 24 below varies dramatically in thickness over the diameter of the spot from X-ray beam 30. Despite this variation, which is difficult to quantify in practice, it has been found that the thickness of the metal layer 26 can still be determined accurately using XRR or RXRR techniques.
Mathematical analysis has shown that these results can be achieved since the X-ray reflection signal primarily comes from interference effects between the upper surface of the metal layer and the boundary between the metal and oxide layers. Very little signal is attributed to the thickness of the oxide layer which, as noted above, varies significantly. Part of the reason that the signal from the oxide layer is so small is that for such a relatively thick layer, any fringes would be so close together that they cannot be seen. In addition, and as noted above, since the reflection coefficient is so small, multiple reflections have a relatively undetectable effect on the X-ray reflection signal. In view of the above, mathematical modeling of the signal from patterned wafers can be simplified allowing the determination of the layer thickness.